This disclosure is directed to a method and apparatus for detection of the percent moisture in a sample. It is particularly adapted to use with flowing materials. In like fashion, it can be used with non-flowing batches. The present invention is directed to a transient NMR test apparatus which includes a coil which interrogates or tests a specified or definite "detection volume." This term refers to the volumetric space within a coil (an RF coil) which is interrogated or tested for resonant response. For instance, a typical configuration involves a pipe, a flat belt or a V-belt, for conducting a flow of some product through the radio frequency magnetic field of the transient NMR detection coil. The coil has a specified volumetric capacity. The detection volume is that portion of material in the pipe; namely, a cylindrical plug within the RF coil and exposed to the requisite steady magnetic field perpendicular to the RF field. The coil is conventionally would as a cylinder, defining a circular cross-section. It is possible to use the entire cross-section such as a filled pipe, or to use less than the entire cross-section as in the instance of a flat belt conveyer passing through the RF coil. The maximum detection volume is thus geometrically configured dependent on the shape and size of the RF detection coil normally positioned around the cooperative pipe. The term "pipe" refers to a tubular member formed of a material permitting magnetic flux and RF field lines to be formed through the pipe. The pipe should be made from non-conducting materials such as plastic or glass. It is not possible too use conductors and ferrous materials because they will not pass the radio frequency field. If a conductive pipe must be used, the detection coil can be mounted inside of the pipe. This also applies to various types of conveyer belts.
The detection volume is that volume within the physically constrained coil and magnetic field described above. Ordinarily, the present invention is applied to a pipe or other type of conduit which is conducting a flowing material. Flow velocities may ordinarily vary widely. Even with a high flow velocity, such as 300 feet per minute, the test which is contemplated herein is accomplished so rapidly that the portion of flowing material within the detection volume is substantially stationary during testing. The flowing material is pumped through the pipe, passing through a magnetic field transverse to the flow. The magnetic field provides polarization of the element of interest. Hydrogen is the element of interest for measurement of water concentration. Periodically, a pulsed RF magnetic field at right angles to the polarizing magnetic field is transmitted from the coil. The pulse has a duration measured in microseconds. If the pulse has a duration of three microseconds, and if the return signal from the NMR interrogation has been completed within 20 microseconds, then the flowing material during the entire 20 microsecond interval moves approximately 0.001 inches at approximately 300 feet per minute. If the interval is 100 microseconds, the movement will be about 0.005 inches. As will be understood, this small movement does not particularly distort the data obtained from the pulsed NMR interrogation.
The present invention may be utilized for measurement of flowing materials wherein the material is formed of hydrogen containing compounds. It also operates successfully where there are no hydrogen compounds. For instance, an important measurement is the moisture content in flowing cement. Cement is a compound essentially free of hydrogen. Therefore, one may safely assume any hydrogen measured in the flowing cement is part of the moisture. An alternate situation is the measurement of moisture in flowing food products such as flour or corn starch. Such food products are primarily hydrocarbons and have various hydrogen compounds in their make-up. Another situation where moisture measurement is in a flow of a material which has been wetted by oil and water or absorbed the oil and water. The oil can be treated as a part of the material undergoing testing and hence the measurement of the percent moisture is analogous to the measurement of water in hydrocarbons material. The oil can be measured separately when the water is treated as part of the material being tested.
Many mixes and variations between the material and the moisture in the material can be imagined. The categories described above are representative of such variations. In the several categories, suitable measurements are obtained whereby the moisture content can be indicated.
An important feature of this disclosure is the ability of the method and apparatus to measure the moisture content without weighing the material. It is inconvenient to rapidly get the weight of a batch of material. This typically requires more time for the scales to settle than the time required to obtain the NMR test data taught by this disclosure. Even worse, there is far greater difficulty in obtaining the weight of a flowing mass. For instance, particulate material such as cement, flour, foodstuffs and the like flow through a pipe in quantities or at rates which vary somewhat. The weight cannot be presumed. It is relatively difficult to measure weight of a flowing material. The present invention avoids the separate measurement of weight. Separate weight measurement is avoided by using an NMR voltage proportional to the total weight of the sensed sample. Another NMR voltage is obtained from the same transient NMR signal which is directly proportional to the weight of water. The ratio of the last voltage to the first (times a constant) gives the water concentration without weighing. Accordingly, data is then obtained indicative of percent concentration of water independent of weight measurement. In a first embodiment of the present invention, the material of interest in which moisture is measured in a material which does not include hydrogen as an element of the material. One example is flowing cement, primarily calcium carbonate.
An alternate embodiment is concerned with the measurement of moisture in materials, where the material itself is a compound which includes hydrogen. Food such as flour, starch, and hydrocarbon products exemplify this category. Another example is cellulose materials such as paper, wood or plants which hold water. Not only is there hydrogen in the cellulose material making up the paper, hydrogen is also in water; the water may be different phases. In very low percentage moisture content, the water very tightly bonded in a crystalline phase. Where there is more water, it is less tightly bonded in an amorphous phase. Higher concentration of loosely bonded water are typically found in capillary spaces in the fiber structure of the paper.
Another embodiment of this invention involves detecting moisture content in a sample wherein oil is also in the sample. In other words, there are tow different liquids present. This invention is best able to separate the transient NMR response of the two liquids provided the relaxation times of the hydrogen in the two liquids are different by approximately threefold or more. This difference provides adequate signal discrimination. The separation is easier with a larger difference. If reduced accuracy is acceptable, it is practicable to work with a twofold difference in T.sub.2. From the foregoing, it will be observed that a variety of materials falling within the broad definitions set forth in exemplary fashion can be measured and tested to determine moisture content as a percentage. While the absolute measurement of the water is seldom used, the preferred presentation is the voltage ratio percentage approach since variations of density and temperature are removed.
This invention may therefore be adapted for use with a flowing stream of materials in the general categories described above. The output can be obtained periodically. For instance, a new moisture measurement can be obtained once per second. The rate at which the data can be obtained usually exceeds the rate at which the date is normally required. A single transient NMR interrogation of the present invention is able to be completed in microsecond speed. Briefly, the flowing material in the pipe is directed through a magnetic field. The magnetic field polarizes the hydrogen nuclei. The flowing material also passes through a coil forming a field, namely an RF field. An interrogation radio frequency pulse is transmitted into the RF coil. A short pulse is sufficient, typically in the range of a few microseconds. A received transient NMR pulse signal encodes the data of interest, the onset of the received NMR signal measured from transmission of the interrogation pulse, being as short as 50 microseconds. Depending on scale, the time after transmission may be longer. A large number of data points can be obtained, but excessive data points are usually not needed because the flowing material does not move very fast between pulses. While the NMR duration material is moving, the rate of speed is usually so slow relative to the transient duration there is usually no benefit from obtaining a large number of data points other than improving the signal to noise ratio. The foregoing sets forth the general nature of this invention and features thereof. The description set forth below directed to the preferred embodiments is best understood in conjunction with the drawings.